The present invention relates to antiwear additives for lubricating compositions and specifically a trisubstituted-borate compound useful as a lubricating composition extreme pressure additive.
Lubricating compositions reduce friction and reduce or prevent destructive contact between moving metal surfaces as long as a film of the composition is maintained between the moving surfaces. This particular type of lubrication is referred to as hydrodynamic lubrication.
Some antiwear additives enhance a lubricating composition's hydrodynamic lubrication. However, when the pressure and/or rubbing speeds between the moving metal surfaces increase, the lubricating film is forced out from between the moving metal surfaces. This results in destructive metal-to-metal contact and wear. Lubrication under these extreme pressure conditions requires an additive that is adsorbed by or reacts with the metal to form an adherent protective film having a lower sheer strength than the metal. The type of lubrication needed under these conditions is called boundary lubrication with additives for this type of lubrication known as "extreme pressure antiwear additives."
Many extreme pressure antiwear additives are known, with the most commercially used additives being phosphorus containing compounds, such as dialkyl dithiophosphates. While these phosphorus-containing compounds provide a high degree of boundary lubrication, there is a move away from them, especially for use with internal combustion engines because of the belief that when these compounds are carried by the exhaust gases they react with and reduce the life of emission control catalysts.
Various types of boron-containing compounds are known as lubricating oil extreme pressure antiwear additives. One type of boron-containing compound antiwear additive is the ester of boron acids or oxides as disclosed in U.S. Pat. No. 2,975,134. Esters of dihydroxy compounds and boric acids or boron oxides are disclosed as useful antiwear additives. The major drawback with these types of compounds is their susceptibility to hydrolysis in the presence of water. Boron-containing compounds resistant to hydrolysis are disclosed in U.S. Pat. Nos. 3,509,054; 3,347,793; 3,359,298; and 3,356,707, wherein the compound is provided with at least one hindered phenol as the ester group. The use of 2-hydroxy-4-(mercaptohydrocarbyl)-1,3,2-dioxaborolanes and alkylammonium bis[(mercaptohydrocarbyl) ethylenedioxy]borates are disclosed as useful antiwear additives in lubricating compositions in U.S. Pat. No. 4,115,286.
While these boron-heterocyclic compounds provide boundary lubrication, they do not approach the boundary lubricating ability of phosphate-containing compositions. Thus there remains the need to find an extreme-pressure antiwear additive that can provide equivalent properties as, or superior properties to, phosphorus-containing compounds in lubricating compositions.